A 13-year-old boy was arrested in Bosnia Wednesday after allegedly shooting and wounding a teacher inside an elementary school building, police and local officials said.

The teacher, a 38-year-old man who also served as the assistant headmaster in Lukavac Elementary School in the northeastern town, sustained serious wounds and was undergoing surgery, police said in a statement. The assailant was detained after the shooting in which no students were injured, the statement added.

Police released no information about the shooter’s motive, but local officials confirmed the boy was expelled from the school earlier this year for unruly behavior. A local politician in Lukavac, Dino Osmanovic, claimed the boy had previously threatened to take revenge for the disciplinary action.

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Osmanovic said he notified the police of the danger in late May, but was told they had taken "necessary measures, that everything was under control and that they cannot share more information about the case because it involves a juvenile."

The shooting comes on the heels of two mass murders in neighboring Serbia where, in early May, a 13-year-old boy took his father’s gun to school in Belgrade and opened fire on his fellow students. The shooting claimed the lives of nine students and a school guard. A day later, a 20-year-old used an automatic weapon to randomly target people in two villages south of Belgrade, killing eight people and wounding 14.

Serbia’s twin mass shootings reverberated around the Balkan region that is awash with small arms and weapons since the 1990s wars, part of the breakup of Yugoslavia, particularly in Bosnia - the country of about 3.5 million people. According to a 2010 study by the United Nations Development Program, there are about 750,000 weapons in illegal possession in Bosnia.